For example, an airframe of an aircraft is configured of a metal material such as aluminum alloy or a composite material such as Carbon Fiber Reinforced Plastic (CFRP), and structure elements are secured to each other by a securing structure part using multiple fasteners. As an example of the securing structure part, a securing structure part is known, in which a through hole is formed in overlapping multiple structure elements, a metal fastener passes through the through-hole, and the multiple structure elements are secured to each other by fastening a nut-shaped collar to the tip part of the fastener.
In addition, a resin cap may be provided to surround the tip part of the fastener and the collar. In a case where the aircraft is struck by lightning, the cap is used to prevent sparks from occurring in a portion between the structure elements and the metal fastener or collar. Particularly, in a case where the tip part of the fastener or the collar is positioned on the inner surface side of a fuel tank, the cap is effectively used.
PTL 1 discloses a securing structure part which uses this cap. In PTL 1, occurrence of sparks is prevented by enclosing gas inside the cap.
In the cap disclosed in PTL 1, since the cap is secured only by coating the outer periphery of the cap with a sealant, the reliability of securing the cap is insufficient, and there is a concern that the cap may fall off.
Particularly, during operation of an aircraft, the aircraft is subjected to severe temperature variation in which the airplane is subjected to high temperatures (for example, 100° C.) on the ground and is subjected to low temperatures (for example, −60° C.) when the airplane is flying. Accordingly, especially in a case where temperature variation occurs, it is necessary to secure the cap such that the cap does not fall off.
Meanwhile, PTL 2 and PTL 3 disclose inventions in which a fastener and a cap are directly screwed to each other to be secured. Specifically, the inventions are disclosed in which a female thread part which is screwed to a male thread part on the tip of the fastener is formed on the inner periphery of the cap, and the cap is secured by screwing the tip of the fastener to the cap.